Human antiviral protein APOBEC3G induces extensive mutations and prevents the accumulation of HIV DNA rendering the virus non-infectious. To evade this host defense mechanism, HIV expresses the virion infectivity factor (Vif). Vif mimics the substrate receptor in a multi-component cellular ubiquitin ligase and recruits the ligase to polyubiquitinate APOBEC3G for proteasome-mediated degradation. Through direct interactions with APOBEC3G and the ubiquitin ligase components ElonginB/ElonginC and Cullin5, Vif enables HIV to escape a key element of the innate immune response. Our overall goal is to establish the structural principles by which HIV Vif sequesters APOBEC3G and to provide structural details of the interactions between Vif and host cellular proteins. We will determine the crystal structures of HIV Vif in complex with its various cellular binding partners as well as a complete complex of Vif/APOBEC3G/ubiquitin ligase. Specifically, we will examine the structures of complexes that contain i) the Vif-ElonginB/ElonginC, ii) the Vif-Cullin5, and iii) the Vif- APOBEC3G interactions. Structural information gained from these studies will provide an in-depth understanding of the mechanism by which HIV Vif hijacks the host ubiquitin ligase to circumvent the innate immune response from APOBEC3G. Further, the structural details will enable the rational design of Vif inhibitors that might lead to new therapeutic interventions for HIV infection.